Schistosomiasis

Schistosomiasis The World Health Organization has targeted a number diseases on which to focus support and research effort. These diseases are: • • • • • • • •

Malaria, Schistosomiasis (bilharzia or "snail fever"), Leishmaniasis, African trypanosomiasis (sleeping sickness), American trypanosomiasis (Chagas disease), Lymphatic filariasis (which leads to elephantiasis), Onchocerciasis (river blindness) Leprosy.

Over 500 million people, almost all of them in the developing countries, suffer from these diseases, which can cause terrible anguish, deformity, and death. At the same time they cause considerable economic losses, and frequently interfere with development projects (particularly water projects such as dams and irrigation schemes, and planned and unplanned forestry). The death toll from the diseases - particularly among african children suffering from malaria - is expected to double by 2010, possibly reaching four million lives a year, unless radical solutions are found. Population increase, the spread of parasite resistance, mass migrations, environmental disturbance, and disruption of control programmes through economic devastation, civil unrest and wars, all contribute to the tropical disease problem. Schistosomiasis, also known as bilharzia or snail fever, infects in excess of 200 million people and results in severe morbidity and mortality. It is principally a disease of tropical and sub-tropical regions and is found in South and Central America, Africa, Asia and south East Asia. There are three main species of schistosomes infecting humans, S. mansoni, S. japonicum which inhabit the mesenteries around the intestine and S. haematobium which are found in the venules surrounding the bladder.

Life cycle

Among the digenea, schistosomes are unusual in that the are dioecious, having separate males and females. They are located in the mesenteric veins. The males are significantly larger than females. The females lie inside a ventral fold in the tegument of the males called the gynecophoral canal. Worms pair as young juveniles in the liver and remain so for life. The eggs which measure 140 x 60 µm, are not operculate, and have a distinctive shape and spine. The position of the spine is characteristic of the species. The eggs are responsible for much of the pathology associated with the disease.

How do the eggs exit to the exterior? The eggs pass through the walls of the mesenteries, and through the intestinal walls into the gut lumen. How they achieve this is still not well understood, however, it is likely to be a result of a number of interacting factors. Physical factors such as the mechanical action of the egg spine, acted upon initially, by the host blood pressure and then by the peristaltic action of the gut help drive the egg into and through the tissues. In addition, the miracidium within the egg has been shown to release proteolytic enzymes which may help it digest its way through the host tissue. The host inflammatory reaction, which is a delayed type hypersensitivity reaction and which forms a granuloma around the egg, also seems to be essential for successful migration of the egg to the lumen of the intestine. Experimental infections in mice given anti-inflammatory agents results in reduced granuloma formation with the eggs becoming trapped in the intestinal tissue. Not all the eggs pass out via the intestine. Many of the eggs are swept back to the liver where they are trapped and form liver granuloma. In general, there is 6 day period between the time the schistosome eggs are laid and they leave the host, by which time they are fully embryonated and ready to hatch. There are three principal factors which are important in the hatching of schistosoma eggs: • • •

Temperature ( 25-30 C), Light, and Osmotic pressure.

On entering a hypotonic environment (water) the increase in the osmotic pressure as water enters the egg and the activation of the enzyme leucine amino peptidase , in hibited by NaCl, results in rupture of the egg shell . (Note: when isolating schistosome eggs from mouse livers the eggs are maintained in isotonic saline, 0,85%, preventing them from immediately hatching) The egg, which is mechanically ruptured along its long axis, releases a highly motile (2 mm/sec) ciliated miracidium which measures approximately 150-180 m in length. (It is noteworthy that the sex of the adult worms which will eventually be produced from the miracidium is already determined at this stage. Therefore, if a snail is infected with a single miracidium all the resulting cercariae will produced adults worms in the definative host which are either all male or all female.) The miracidium searches out and penetrates the snail intermediate host. It can remain infective for 8 - 12 hr. To increase the chance of the miracidia locating the host, it has a negatively geotactic and positively phototactic behavioural response which tends to place it in the general environment of the snail host, Biomphalaria glabrata. Chemical attractants from the snail such as mucus, long chain fatty acids and even amino acids attract the miracidia. Recently, it has been discovered that amines such as dopamine are also highly attractive to the miracidia.

After the miracidia makes contact with the snail there is a period of exploratory behaviour prior to penetration. Seventy percent of the miracidia appear to penetrate through the foot of the snail, other penetration sites include the tentacles and the edge of the mantle. Penetration is a combination of mechanical motion of the apical papillae and histolytic secretions released from the penetration glands. The cilia are not lost until after penetration is complete. The location of the next stage within the snail, the sporocyst, is dependent on the schistosome species. • • •

S. mansoni - at the site of penetration, usually the foot. S. haematobium - same as above S. japonicum - there is a preference for cavity organs , viscera and heart.

The sporocyst undergoes development and produces by 35-600 daughter sporocysts after about 3 weeks. The daughter sporocysts migrate to the digestive glands of the snails and produce the next infective stage, the cercaria. The mean cercarial output from an infected snail has been estimated to be about 1500/day and this last for up to about 18 days. The period from penetration of the snail to release of the cercariae is about 4 weeks. The cercariae are released on a circadian (24 hr) cycle during daylight hours. It has been determined that the pattern of cercarial shedding is dependent on the focus of the infection and the behaviour of the host population. In one study in Guadeloupe island, where there was both a human and a sylvatic focus of infection, three different shedding patterns were obtained in different regions. In the urban area, the human population were the focus of infection, cercariae were shed early in the morning. In a remote country area where the focus of the infection cycled through sylvatic hosts the cercarial shedding pattern occurred in late afternoon. However, in a rural community where there both humans and rodents acted as hosts, the shedding pattern was found to be intermediate to the other two. The cercaria are very active, non-feeding stages which rely on stored glycogen food reserves. These reserves appear to vary considerably (4.8 - 14.2 ng of glycogen /cercariae) and may depend on the nutritional state of the snail and level of infection. Glycogen reserves in the cercaria have been shown to decline in a negative exponential manner with time after shedding. The rate of loss is, not surprisingly, greater in the tail than the rest of the cercarial body. Host location Once released from the snail, periodic bursts of activity keep the cercariae just under the surface of the water. As well as these endogenously driven bursts of activity, sudden shadows also evoke cercarial activity. Once in close contact with the next host, products released from the skin have a dramatic effect on the cercarial behaviour causing

continuous swimming motion interspersed with frequent reversals which is a typical example of what is known as klinokinetic behaviour. Penetration Penetration into the host skin has three distinct phases: 1. Attachment 2. Creeping over the skin surface - exploratory behaviour Both these behaviours are triggered by chemical and thermal stimuli. Penetration into the epidermis - This is in response to chemical stimuli, products are aliphatic hydrocarbons, probably free fatty acids, on the skin produced by bacterial esterases working on tri-glycerides. The actual penetration is a combined mechanical and secretory process the initial phase of which may take as little as a few minutes. As penetration proceeds there are structural and physiological changes which occur and accompany the transformation of a freeliving infective stage to a parasitic larval schistosomulum. Structural changes - these take less than 1 hr. 1. Change in outer membrane. The cercarial tegument is bounded by a classic trilaminate plasma membrane which has a 1-2 m thick glycocalyx. This outer surface coat is mostly shed with the tail at the time of penetration. 2. The various penetration glands empty. 3. The Tegumental cell bodies which lie beneath the muscular layer release membranous vesicles which pass into the tegumental cytoplasm and form a multilaminate tegumental surface, replacing the tri-laminate membrane. 4. In addition, oesophageal glands release their contents into the oesophageal lumen heralding the start of feeding. This transformation process requires two physiological triggers, elevated temperatures and iso-osmotic conditions. The glycocalyx is thought to control the surface permeability in fresh water and its loss coincides with osmotic sensitivity. Biochemical changes As well as physical changes there are a variety of biochemical changes which also take place during this transformation from free-living to parasitic form. There is a switch in the larval energy metabolism over the first 24 h from an aerobic glycogen base metabolism to a predominately anaerobic one which is accompanied by an increase in lactate production. Also during this first 24 h, there is a remarkable turnover of surface molecules on the schistosomula surface. As time passes there is the appearance of more surface molecules which have low reactivity, in terms of stimulating the hosts immune

system. By 24-48 hours the schistosomula has become completely refractory to antibody mediated immune cell cytotoxicity. In addition to providing molecules on the surface which mask antigenic epitopes, the schistosomula surface also has the ability to coat itself with host molecules. Host erythrocyte surface glycolipids are adsorbed onto the schistosomula surface helping to mask sensitive parasite epitopes from the host defenses system. The parasite enters the initial epidermal layer of the skin very rapidly (less than 30 min) and then come to rest when they reach the dermis which appears to present a temporary barrier to further penetration. They remain at this location for about 40 hours. Once through the dermis they locate a venule within about 10 hours and require a further 8 hours to penetrate the venule wall. Once in the blood capillaries the schistosomula are carried to the first capillary bed, the lungs, where they become lodged and double in size over the next few days. This so called lung phase lasts from 3-8 days. Following this period the larvae make there way to the liver, the exact root is unknown. Upon reaching the liver the schistosomula mature to young adults pairing between 28-35 days postinfection. When the worms are mature the paired adults migrate out of the liver to the mesenteries where the female begins egg laying. The male and female worms remain in close association, the slender female lying in a ventral groove on the male surface called the gynecophoral canal. This is an intimate association with the female receiving products, such as glucose, trans-membranously from the male. The importance of the association is highlighted by the fact that if the female fails to mate it does not mature properly and remains stunted. It has been estimated that in human infections adult worms can survive in the host for 20-30 years. Parasite success rate In the mouse model system only about 20% of the initial cercarial inoculum makes it to the adult stage. Although there has been much controversy over the site of attrition of the larval worms, it now seems clear that the greatest loss of larval stages occurs during the migration through the lungs, with relatively small losses during migration through the skin. Fate of the Schistosome Eggs Although many of the eggs pass through the gut mucosa and exit the host with fecal material, as many as 50% of the eggs can be swept by the blood stream back to the liver, where they become lodged in the liver parenchyma. The lodged eggs cause the host inflammatory response leading to granuloma formation and liver fibrosis, and is the chief cause of pathology. Pathology of Schistosome Infections The pathology associated with a schistosome infection arises primarily from the schistosome eggs ( single sex infections, thus no egg production , cause little pathology), both those in the intestine and those swept back by the blood to the liver, and on occasion

to other tissues such as the brain and lungs. At least 50% of the eggs laid fail to reach the exterior. Clinical signs of the disease take on different phases: • • •

Acute Chronic - subclinical Severe - symptomatic

Acute Schistosomiasis The acute stage, often known as Katayama fever, is normally found in young children or young adults with no previous exposure to the disease, and is particularly prevalent in individuals with S. japonicum infections. The acute reaction is in response to the sudden high level of antigen exposure and is usually associated with the onset of egg deposition. Clinical symptoms consists of skin rashes, asthma-like episodes, daily fever, malaise, diarrhoea, swollen lymph nodes and aching joints and a number of other non-specific symptoms. Pathologically one sees large florid granuloma with a rich infiltration of eosinophils. Frequently, heavy infections can lead to fibrotic chronic schistosomiasis or the death of the patient. Chronic Schistosomiasis In chronic schistosomiasis the patient experiences diarrhoea and fevers. In children the infection can depress their growth rate. The infection also leads to enlargement of the liver and spleen. Fibrosis of the liver can result in portal hypertension, hepatosplenomegaly, ascites formation, oesophageal varices leading to fatal hematemesis ( vomiting blood). In S. haematobium, fibrosis of the bladder may lead to ureteric obstruction, pyelonephritis and hydronephrosis leading to renal failure. Granuloma formation

After egg deposition, which occurs predominantly in the periportal area, the egg, over about a 16 day period becomes surrounded by a dense infiltrate composed of mainly lymphocytes, macrophages and a variable number of eosinophils, held together in a extracellular matrix. In athymic mice such lesions do not occur, suggesting that there is a strong T-cell regulation of the granuloma response. Whereas mice with impared B-cell function still exhibits normal granuloma formation. Therefore Granuloma formation is a T-cell dependent and t-cell mediated process. Eggs measure up to 70 m in width and therefore cannot traverse the capillary beds as the blood flows through the liver. Granuloma size and cell composition vary depending on the schistosome species, host species and the intensity and duration of the infection, and even the tissue location can have an impact on granuloma size. The principal factor however, is how immunoresponsive the host is to the schistosome egg antigen. Although some of the tissue response is due to the physical presence and damage by the egg, the majority of the pathology is due to the host response to the soluble egg antigens which are released through submicroscopic egg shell pores. In normal hosts, reactivity to SEA - soluble egg antigens, peaks early, producing large florid lesions but as the infection becomes more chronic, i.e. by 8-10 weeks post infection, granulomas tend to become relatively smaller due to a modulation of the host hypersensitivity response. Activated T-helper cells are instrumental in the induction of IL2, which is the principle cytokine required for the formation of normal granulomas. Down regulation of the production of this cytokine is initiated by a subset of suppressor-inducer T-cells. Modulation of these granulomas is immunologically complex. To summarize, immunoregulation of the granulomas requires multiple effector systems working in concert, which achieves a deceptively simple host adjustment to the persistent generation of parasite antigens. This modulation can be regarded as beneficial to the long term maintenance of the adult parasite and its life cycle. It is obvious a delicate balance. If there was no immuno-depression it would lead to a rapid death of the patient. Complete immunosuppression on the other hand results in low egg excretion and eggs with defective metabolism and also leads to high host mortality. Concomitant Immunity Concomitant immunity has long been considered a feature of schistosome infections and describes the phenomenon where by the adult worms can survive happily in the mesenteric veins where as the host seems to be resistant to secondary infection. What is responsible for this type of immunity? Experimental evidence in the mouse-schistosome model suggested that concomitant immunity was due to non-immune processes. Wilson and co-workers suggested the hepatic shunt theory. As previously mentioned, as an infection proceeds there is a gradual build up of eggs in the liver which clogs the intra-hepatic circulatory system.

These workers found that as a result of this blockage both intra and extra hepatic astamosis occurred such that the blood flow was shunted around these obstructions. These by-passes usually occurred in vessels of larger diameter such that the schistosomula on leaving the lungs were swept past the liver, where they would normally be trapped by the capillary bed and complete their development, and instead passed to other sites and eventually died. The researchers demonstrated this by injecting polystyrene beads of known diameter and showing that they ended up in sites other than the liver, once a critical threshold number of eggs were deposited in the liver. However, whether this occurs in human infections, or is a peculiarity of the mouse model system, is not clear. Given the enormous size of the human liver in comparison to the mouse and therefore its greater capacity to sustain higher egg burdens it seems less likely that this would be the only mechanism resulting in the concomitant immunity. Epidemiological studies have demonstrated a pattern of age related immunity to reinfection which is not consistent with that which would be observed with normal nonsterile immunity. The bell shaped curve on the age-prevalence curve shown below suggests resistance due to acquired immunity or as a result of a change in the pattern of water contact.(It is well known that in general, children tend to play in water and therefore have more frequent contact with infective stages than the adult population.)

There is some evidence for age related resistance to schistosomiasis: 1. Some heavily infected individuals, after treatment do not become re-infected. Resistance is not demonstrable in children under 10 years but increases progressively with age. 2. Some studies have demonstrated an association between of high levels of eosinophilia and resistance to re-infection. 3. Children under the age of 10 are readily re-infected after treatment, demonstrating a lack of resistance. This is as a result of increased levels of circulating IgM antibody which bind to the schistosomula, and as a result, blocks or masks the binding site for IgG antibody. IgG is important for initiating antibody-dependant cell-mediated cytotoxicity by eosinophils and macrophages. The level of IgM production decreases as the child ages which coincides with increase resistance to infection. Diagnosis The most obvious method of diagnosis is the identification of eggs in the stool for S. mansoni and S japonicum or in the urine for S haematobium. For these types of diagnosis a simple faecal smear is often inadequate, therefore, some type of egg concentration technique is require. If no eggs are found, but the outward symptoms are suggestive of schistosomiasis, then rectal, liver or bladder biopsies may be necessary. An intradermal skin test using schistosome antigen has been found to be a fairly sensitive immunological test. But this test will not show positive until the patient has been infected for 4-8 weeks. The circum-oval precipitation test has also been found to be fairly sensitive and is particularly useful for determining if treatment has been successful, because the test becomes negative after all the eggs in the tissue have been killed. The test depends on observing the formation of a precipitate around isolated schistosome eggs in the presence of sera from infected individuals. Treatment and Control Drug treatment is still the principal method of control and the drug of choice is praziquantel. However the degree of recovery from the infection depends on the extent of the damage caused by the infection. If extensive fibrosis has occurred this cannot be reversed and therefore there is permanent liver damage. In addition the efficacy of praziquantel seems to depend on the patients prior exposure to the disease. Recently it has been found that certain populations, not previously exposed to schistosomiasis, respond poorly to the praziquantel treatment, their immune systems are not primed. It seems that the mode of action of praziquantel is such that to be fully effective it requires the participation of the patients immune system.

The control of schistosomiasis other than with the use of drug therapy is difficult and as yet there is no effective vaccine. Other methods of control which have been tried are: • • • • •

Drainage of marsh areas where snails breed Improved sanitation Education The use of molluscicides Introduction of bio-control agents, such as predatory snails

Swimmers Itch Many species of flukes belonging to the family schistosomatidae are parasitic in birds and in other animals, and many snails harbour the developmental stages of these parasites. Cercaria from various different species of these bird schistosomes (Trichobilharzia, Gigantobilharzia, Ornithobilharzia) may be found in large numbers in freshwater streams ponds or lakes in North America. These cercaria often attempt to penetrate vertebrates which are not the normal hosts. The penetrating cercaria do not survive but produce and intense inflammatory reaction resulting in dermatitis with intense itching often known as swimmers itch.